1. Field of the Invention
This invention relates to information recording and more particularly, to high density information record of a variable electrostatic capacitance type, such as video or digital audio discs, on which an information signal is recorded as geometric variations.
2. Description of the Prior Art
In certain information playback systems of the electrostatic capacitance type, an information signal is recorded as geometric variations by forming pits in a spiral plane or groove or in concentric planes or grooves, thereby obtaining high density information records such as video or digital audio discs. The individual pits of the record are very small and are arranged in a very high density. When the geometric variations are tracked with a pickup stylus such as of diamond having an electrode, capacitive variations are established between the stylus and the record according to the geometric variations, so that the recorded information signal can be played back.
Several kinds of information signal records of the electrostatic capacitance type have been proposed for use in the above type of electrostatic capacitance playback system. In one such record medium, an electrode is provided on one or both surfaces of a record substrate on which geometric variations are press molded according to the information signal. This permits formation of an electrostatic capacitance to be established between the record electrode and an electrode of a pickup stylus. More particularly, the record substrate is covered with a thin metallic film having a thickness of several hundred angstrom on which pits are formed as desired, and also with a several hundred angstrom thick dielectric layer overlying the metallic film. The dielectric layer serves to prevent short circuiting of the electrodes and increase the dielectric constant between the electrodes. The record of this type needs a number of fabricating steps including press molding the record substrate, depositing the thin metallic film and the dielectric layer, and the like. Thus, the fabrication is complicated and troublesome with the need of a relatively large-scale manufacturing apparatus. Thus, the production cost becomes very high.
Another type of known information record of the electrostatic capacitance type is one which is obtained by press molding a conductive plastic composition comprising a resin such as polyvinyl chloride and several ten percent of carbon black so that the information signal is recorded as geometric variations. In this type of record, an electrostatic capacitance is established between the electrode of a pickup stylus and the record itself. Thus, the step of depositing a thin metallic film on a substrate is not needed. Because fine particles of carbon black are individually covered with the resin, the dielectric film is not necessary. Thus, this type of record can be simply manufactured at a relatively lost cost.
However, when the record made of the conductive resin composition comprising carbon black is set in a playback system and played back over a long term, its surface is worn by means of the pickup stylus contacting therewith. This results in deformation of the geometric variations or pits formed on the record, causing capacitive variations to vary. Thus, reliable playback is not possible. Especially, when the record is a video disc record and is subjected to still reproduction or freeze frame, a given portion of the disc is repeatedly contacted with the pickup stylus at a pressure of 400 to 500 kg/cm.sup.t2 at a rate of 15 times/second with the serious problem that the given portion is worn. In addition, the conductive resin compositions are not readily moldable.
A further type of information record of the electrostatic capacitance form was proposed in order to solve the problems described above with regard to the moldability and disc wear, in which the resins contained in the conductive compositions have improved properties. More specifically, Japanese Laid-open Patent Application No. 57-177047 describes a video disc which is obtained by molding a conductive resin composition comprising a mixture of a graft polymer obtained by subjecting 99 to 76 parts by weight of vinyl chloride to graft copolymerization with 1 to 15 parts by weight of a copolymer of 10 to 65 wt% of ethylene and the balance of vinyl acetate and 5 to 30 wt% of carbon black. This conductive composition is advantageous in that various characteristics such as heat resistance, thermal stability, impact resistance, melt fluidity and the like are very good. However, this conductive composition was not always satisfactory as a molding material for video discs of the electrostatic capacitance type. The video discs obtained from such compositions are relatively poor in wear resistance and have the drawbacks that when such a disc is played back or reproduced, there may involve a lowering of reproduction output, deterioration of image quality and defects of pictures due to a failure in contact between the video disc and the pickup stylus. Such video discs have thus little commercial values.